1. Field of the Invention
The present invention relates to a liquid crystal display device capable of displaying gray scale images.
2. Description of the Related Art
A display principle using a ferroelectric liquid crystal device is proposed by N. A. Clark, et al. (See xe2x80x9cApplied Physics Lettersxe2x80x9d Vol. 36, No. 11 (published on Jun. 1, 1980), Japanese Laid-Open Publication No. 56-107216, U.S. Pat. Nos. 4,367,924, and 4,563,059.) The ferroelectric liquid crystal device is a device in which the ferroelectric liquid crystal material is sealed into an extremely thin cell. A display mode using the ferroelectric liquid crystal device is generally referred to as Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) mode. This display principle provides a significantly quick response property, e.g., tens of microseconds, and a wide angle of view. Thus, various researches and developments have been carried out on the SSFLC mode.
The SSFLC mode as described above is only capable of displaying black/white two-tone images, i.e., incapable of displaying gray scale images. In order to address such a drawback, various improvements have been made. Two typical examples among the improvements, the area-division gray scale driving method (see, for example, Japanese Laid-Open Publication No. 8-50278) and the time-division gray scale driving method (see, for example, Japanese Laid-Open Publication No. 6-18854), will be described.
The area-division gray scale driving method is performed such that a single pixel is divided into a plurality of subpixels, and the plurality of subpixels are driven separately.
As one of the simplest examples, consider the case where a single pixel is divided into two subpixels. In this case, it is possible to drive the first and second subpixels separately. Based on the combination of the on/off states of the two subpixels, the brightness of an image can be expressed in four gray levels, i.e., 0, 1, 2, and 3. Thus, the combination of the plurality of subpixels realizes display of gray scale images even in the SSFLC mode.
The time-division gray scale driving method is performed such that one frame (which is a minimum display period) is divided into a plurality of periods, and the plurality of periods are driven separately.
As one of the simplest examples, consider the case where one frame is divided into two periods (subframes). In this case, it is possible to drive the first period (first subframe) and the second period (second subframe) separately. Based on the combination of the first period and the second period, the brightness of an image can be expressed in four gray levels of 0, 1, 2, and 3. Thus, dividing one frame into a plurality of periods realizes display of gray scale images even in the SSFLC mode.
As a matter of course, the area-division gray scale driving method and the time-division gray scale driving method can be used in combination. For example, a pixel is driven with an area-division ratio (=1:the number of subpixels in a pixel) of 1:2 and with a time-division ratio (=1:the number of subframes in a frame) of 1:4:16:64, thereby achieving the display of images in 256 equally-divided gray levels.
The ferroelectric liquid crystal material, which itself is only capable of displaying two-tone images, can display gray scale images by using the area-division gray scale driving method or the time-division gray scale driving method.
However, in the area-division gray scale driving method and the time-division gray scale driving method as described above, the gray scales are digitized (quantitized) by nature, and the number of gray levels is thus limited. Therefore, the area-division gray scale driving method and the time-division gray scale driving method do not have a smooth gray scale property with respect to image signals. Furthermore, in the area-division gray scale driving method and the time-division gray scale driving method, adjustment of the image quality such as xcex3 correction cannot be performed. That is, a gray scale property of displayed images is limited by a predetermined area-division ratio and time-division ratio.
In the area-division gray scale driving method and the time-division gray scale driving method, one of the measures for increasing the number of gray levels is to increase the number of subpixels in a single pixel. This causes some disadvantages that a wiring structure and a pixel structure of the device become complicated, that an effective pixel area (numerical aperture) decreases, etc.
The number of gray levels also can be increased by increasing the time-division number. However, this is technically difficult to achieve because this causes an increase of the driving frequency and requires higher response speed for the liquid crystal material.
According to one aspect of the present invention, a liquid crystal display device includes: a main substrate having a main electrode; a counter substrate having a counter electrode; a liquid crystal material interposed between the main substrate and the counter substrate; and a control section for controlling a response start time of the liquid crystal material by a potential difference between a main electrode voltage that is applied to the main electrode during one frame and a counter electrode voltage that changes in a substantially continuous manner during the one frame, and for changing a transmissivity of the liquid crystal display device based on a magnitude of the main electrode voltage.
In one embodiment of the present invention, the control section changes the transmissivity of the liquid crystal display device by changing a direct-current voltage component of the main electrode voltage that is applied to the main electrode.
In another embodiment of the present invention, the control section changes the transmissivity of the liquid crystal display device by changing a direct-current voltage component of the counter electrode voltage that is applied to the counter electrode.
In still another embodiment of the present invention, the control section changes the transmissivity of the liquid crystal display device by changing a slope angle of a waveform of the counter electrode voltage with respect to the passage of time.
According to another aspect of the present invention, a liquid crystal display device includes: a TFT substrate having thin film transistors arranged in a matrix; a counter substrate having a transparent electrode; a liquid crystal material interposed between the TFT substrate and the counter substrate; and a control section for applying a voltage that changes in a substantially continuous manner to a storage capacitor line during a period within one frame, thereby writing a drain voltage in a drain electrode during the one frame, wherein a response start time of the liquid crystal material is determined by a magnitude of the drain voltage, and a transmissivity of the liquid crystal display device is determined by a potential difference between a potential of the storage capacitor line and a potential of the drain electrode.
In one embodiment of the present invention, the storage capacitor line is connected to a power supply which can be controlled independently of the power supply for applying the voltage to the counter substrate.
In another embodiment of the present invention, an odd-numbered line storage capacitor line to be connected to pixels of odd-numbered lines is connected to a first power supply; an even-numbered line storage capacitor line to be connected to pixels of even-numbered lines is connected to a second power supply: and the first power supply is different from the second power supply.
In still another embodiment of the present invention, the one frame includes a period during which a voltage for resetting an orientation of the liquid crystal material to the initial state is applied to the storage capacitor line.
In still another embodiment of the present invention, the control section applies the voltage for resetting the orientation of the liquid crystal material to the initial state to the liquid crystal material through a source line during the one frame.
In still another embodiment of the present invention, the one frame includes a period during which at least one of the voltage to be applied to the counter electrode and the voltage to be applied to the main electrode resets the orientation of the liquid crystal material to the initial state.
In still another embodiment of the present invention, during the one frame, after a signal has been written in the thin film transistor element, the voltage that changes in a substantially continuous manner is applied to the storage capacitor line.
In still another embodiment of the present invention, the one frame includes a period during which a signal is written in the thin film transistor element, a period during which a voltage to be applied to the storage capacitor line changes in a substantially continuous manner, and a period during which the orientation of the liquid crystal material is reset to the initial state.
According to still another aspect of the present invention, a liquid crystal display device includes: a main substrate having a main electrode; a counter substrate having a counter electrode; a liquid crystal material interposed between the main substrate and the counter substrate; and a control section for applying a voltage that changes in a substantially continuous manner to the liquid crystal material for a predetermined period within one frame, wherein a response start time of the liquid crystal material and a transmissivity of the liquid crystal display device are changed by changing an amplitude of the voltage.
According to still another aspect of the present invention, a liquid crystal display device includes: a TFT substrate having thin film transistors arranged in a matrix; a counter substrate having a transparent electrode; a liquid crystal material interposed between the TFT substrate and the counter substrate; an element having a resistance value that changes in accordance with a voltage applied to a memory capacitor of a pixel; and a control section for applying a voltage that changes in a substantially continuous manner to the element during a period within one frame, wherein the liquid crystal material is connected in series to the element, and a response start time of the liquid crystal material and a transmissivity of the liquid crystal display device are changed by changing an amplitude of the voltage that changes in a substantially continuous manner.
In one embodiment of the present invention, the control section includes a first power supply for applying a voltage to the liquid crystal display material through an odd-numbered line and a second power supply for applying a voltage to the liquid crystal display material through an even-numbered line; and the first power supply is different from the second power supply.
In another embodiment of the present invention, the one frame includes a period during which a voltage for resetting an orientation of the liquid crystal material to an initial state is applied to the liquid crystal material.
In still another embodiment of the present invention, the one frame includes a period during which a voltage for resetting an orientation of the liquid crystal material to an initial state is applied to the element.
In still another embodiment of the present invention, the control section applies a voltage for resetting to an initial state an orientation of the liquid crystal material to the liquid crystal material through a source line.
In still another embodiment of the present invention, during the one frame, after a signal has been written in the thin film transistor element, the voltage that changes in a substantially continuous manner is applied to the element.
In still another embodiment of the present invention, the one frame includes a first period during which a signal is written in the thin film transistor element, a second period during which a voltage to be applied to the element changes in a substantially continuous manner, and a third period during which the orientation of the liquid crystal material is reset to the initial state.
In still another embodiment of the present invention, the liquid crystal material is a ferroelectric liquid crystal material.
In still another embodiment of the present invention, the liquid crystal material is an antiferroelectric liquid crystal material.
In still another embodiment of the present invention, the liquid crystal material has a liquid crystal mode including two or more stable states.
In still another embodiment of the present invention, the liquid crystal display device includes a light source which is off for a predetermined period within the one frame.
In still another embodiment of the present invention, the liquid crystal display device includes a red light source, a green light source, and a blue light source, and field serial color display is performed by sequentially switching from one to another among the light sources for each frame, whereby a single color image is obtained from a plurality of frames.
In still another embodiment of the present invention, the control section controls a waveform of a voltage to be applied to the storage capacitor line, thereby adjusting at least one of an adjustment property and an adjustment balance.
In still another embodiment of the present invention, the control section controls a waveform of a voltage to be applied to the liquid crystal material, thereby adjusting at least one of an adjustment property and an adjustment balance.
In still another embodiment of the present invention, the control section controls a waveform of a voltage to be applied to the element, thereby adjusting at least one of an adjustment property and an adjustment balance.
In still another embodiment of the present invention, the control section adjusts a source signal voltage to be written corresponding to a gray scale signal, thereby adjusting at least one of an adjustment property and an adjustment balance.
In still another embodiment of the present invention, the control section adjusts a source signal voltage that is to be written in the element and that corresponds to a gray scale signal, thereby adjusting at least one of an adjustment property and an adjustment balance.
Now, functions of the present invention will be described.
The liquid crystal display device of the present invention can display gray scale images by controlling the response start time of the liquid crystal material in an analog manner. A portion of a voltage to be applied to the liquid crystal material changes with the passage of time, thereby controlling the timing of reaching the threshold voltage at which the response of the liquid crystal material occurs.
Alternatively, the timing of reaching the threshold voltage at which the response of the liquid crystal material occurs is controlled by adjusting an amplitude of a voltage to be applied to the liquid crystal material, which includes components that changes with the passage of time.
Thus, the invention described herein makes possible the advantage of providing an analog-like gray scale property by changing the voltage even in the liquid crystal modes wherein it is difficult to perform an analog gray scale display, for example, a liquid crystal mode only capable of displaying two-tone images (such as a ferroelectric liquid crystal device, an antiferroelectric liquid crystal device, a cholesteric liquid crystal device having an appropriately adjusted orientation, a nematic liquid crystal device, etc.).
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.